Embodiments of the invention relate generally to magnetic resonance (MR) imaging and, more particularly, to correcting high order eddy-current-induced distortion in diffusion-weighted echo planar imaging.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
It is well known that Diffusion-Weighted Echo Planar Imaging (DW-EPI) often suffers from diffusion encoding direction dependent distortions due to diffusion gradient generated eddy current field. These distortions, if not corrected, can lead to mis-registration among DW images of different directions and inaccuracies in any post processing operations involving DW image combination. Dual spin echo (also called twice refocused) DW-EPI has been proposed to provide a certain level of inherent eddy current cancellation, but with a significant increase in echo time and decrease in signal-to-noise ratio (SNR). For example, a typical dual spin echo protocol may generate about half as much SNR as the corresponding single spin echo (also called Stejkal-Tanner sequence) protocol on liver imaging at 3 T. In many cases (e.g., whole body DW-EPI), increasing NEX is not an option to increase SNR because of the associated increase in scan time. Therefore, it is desirable to keep single spin-echo while reducing the resulting distortion in practice.
Conventional distortion correction methods have focused on correcting only the linear and constant eddy currents (also called B0 eddy currents), either by pre-emphasis or by explicitly modifying gradient waveforms and receive frequency. However, uncompensated eddy currents of high spatial order due to gradient coil leakage field, or simply high order eddy currents (HOEC), can also be significant with the desire for increased b values and the increase of gradient amplitude and slew rate in modern MR scanners. Because of the high spatial order, distortions generated by the magnetic fields created by these eddy currents are not only diffusion gradient direction dependent, but also slice dependent.
It would therefore be desirable to have a system and method capable of correcting distortion due to HOEC in DW-EPI.